In addition, to quantify the scale of the variability, we use the
fractional root mean squared variability amplitude \(F_{var}\) 
[1]}, [2]}. This variable is the square
root of the excess variance after normalisation by the mean count rate
\(\bar{x}\) . This is used as an estimator of the intrinsic source variance and
is defined as:
\(F_{var} =\sqrt{\frac{S^2 - \overline{\sigma _{err}^{2}}}{\bar{x}^2} },\) 
